US 2003/0016205-A1 discloses a lighting unit for use as a backlight of a liquid crystal display device. The backlight is locally turned on, for part of the frame period only, to reduce smear effects occurring for moving images. Such a backlighting is usually referred to as scanning backlighting. The lighting unit comprises a plurality of light sources and associated light-emitting regions that are arranged in the vertical scanning direction of the liquid crystal display. Thus, in the direction in which the multiple gate lines, which select rows of pixels of the display, are driven sequentially. The light emitting sources associated with the light-emitting regions are sequentially turned on and off synchronously with the scanning of the lines of pixels. A light sensitive element is associated with each one of the light-emitting sources. The light sensitive element feeds-back the luminance of the associated light-emitting source to a control circuit which changes the drive signal supplied to the light-emitting source to minimize the difference in luminance between the respective light-emitting regions.
Thus, the scanning backlight produces instead of a constant light plane for constantly illuminating the complete matrix display, light areas which are present for a relatively short period in time only. The relatively short period is shorter than a frame period. This has the advantage that the integration by the human eye which tracks a moving object decreases and thus the smearing becomes less visible. Further, the switching periods wherein the pixels of the matrix display change their optical behavior can be selected to occur when no light is impinging. Usually, in a scanning backlight, the light of a particular one of the light sources has to be concentrated in the associated one of the light-emitting regions; the light should not be divided over the complete area of the matrix display. Consequently, differences in the luminance of the light sources will become quite visible.